1. Field of the Invention
This invention relates to an apparatus and method for treating organic waste sludge and, more particularly, to an apparatus and method for treating sewage sludge.
2. Description of the Related Art
The efficient and commercially effective utilization of organic waste sludge such as sewage sludge is important for society, particularly with the increase of population and demands which are made on land. At present, sewage sludge is dried in large bed pans, and the dried product is used as a compost high in organics. Although such composts are useful in enhancing the retention and storage of water in soil, the composts are low in inorganics and, hence, have limited fertilizer properties. The drying process is environmentally undesirable in that offensive odors are produced. Further, the prior art methods of drying the sludges are capital-intensive.
Processes have been suggested for treating sewage sludge to sterilize and disinfect the same. For example, South African Patent No. 89/6160 discloses such a method which involves treating sewage sludge with anhydrous ammonia gas to increase the pH of the sludge to at least 10, followed by using sufficient inorganic acid to neutralize, or substantially neutralize, the sewage sludge/ammonia admixture. The resulting product is a liquid which is said to be useful as a fertilizer. It is further suggested that the liquid can be dried, e.g., by evaporation.
A prior art process for treating organic material such as sewage sludge for use as a fertilizer is disclosed in U.S. Pat. No. 5,393,317, and it is believed that the process described in the '317 patent is less than desirable due to the time required for treating the same inasmuch as in certain steps thereof, the mixture is allowed to stand for at least 20 minutes. Another prior art process for treating organic material such as sewage sludge is disclosed in U.S. Pat. No. 4,743,287. It is also believed that the '287 process is less than desirable, since water must be added to the mixture of organic material and major elements to produce a moisture content of 12%–30% by weight. Still another process is disclosed in U.S. Pat. No. 5,443,613. It is also believed that the process of the '613 patent is less than desirable, since water must be added to an acidified suspension. It is believed that those processes requiring an addition of water to the product increase the amount of drying required and, hence, causes an increase in drying time.